As on-chip dimensions decrease, the dimensions of the solder connections connecting the chip to the package must also decrease. For small solder connection spacing (e.g., 100 um or less), there is a risk of neighboring solder connections, e.g., solder bumps, shorting together. This is especially likely if there is poor adhesion between the underfill and the on-chip passivation (polyimide). For example, adhesion loss at the underfill-polyimide interface often occurs at the edge of solder bumps, which can lead to delamination and formation of small voids. This weak adhesion is due to a contamination of the polyimide surface by the solder or due to a pre-existing void associated with the undercut of the ball limiting metallization (BLM) structure.
During subsequent reflow of the solder connections, molten solder can get pushed into the voids. These solder extrusions have been related to fails due to shorts. For example, solder extrusions have been observed in high density corners or at high density edges. In fact, yield loss has been found to be up to about 5%.